A purification catalyst used for exhaust gas discharged from a gasoline engine includes a three way catalyst that achieves oxidation and reduction of carbon oxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) at the same time. The three way catalyst mainly includes platinum (Pt), palladium (Pd), and rhodium (Rh) as catalyst metals. Various types of such a catalyst have been proposed including: a catalyst in which the three types of catalyst metals are mixed in a single catalyst layer; a catalyst in which the catalyst layers as upper and lower layers are provided and the catalyst metals are separately supported by the layers; and a catalyst divided into an upstream side section and a downstream side section, in an exhaust gas flow direction, with the catalyst metals separately supported by the sections.
For example, Japanese Patent Application Lain-open No. 2015-24381 discloses an exhaust gas purification catalyst that is capable of efficiently purifying exhaust gas including saturated hydrocarbon and includes: a catalyst layer including only Pt as the catalyst metal on the upstream side in the exhaust gas flow direction; and a catalyst layer including Pd and Rh but including no Pt on the downstream side in the exhaust gas flow direction. In the catalyst layer including only Pt, a center portion includes a larger amount of Pt than a circumference edge portion.
Japanese Patent No. 5287884 discloses an exhaust gas purification catalyst that can suppress grain growth of a catalyst metal and includes: a substrate; a lower catalyst layer that is formed on the substrate and includes at least one of Pd and Pt; and an upper catalyst layer that includes Rh. An area including no upper catalyst layer is provided on the upstream side in the exhaust gas flow direction. The lower catalyst layer includes an upstream stage lower catalyst layer on the upstream side in the exhaust gas flow direction and a downstream stage lower catalyst layer on the exhaust gas flow direction. The upstream stage lower catalyst layer includes an oxygen absorbing and releasing material.